In recent years, attention has focused on a hybrid vehicle and an electric vehicle as environmentally friendly automobiles. A hybrid vehicle is an automobile that uses a motor to be driven by a DC power source via an inverter as a power source in addition to a conventional engine. Specifically, the power source is secured by driving the engine, and further, a DC voltage supplied from the DC power source is converted into an AC voltage by the inverter, and then, another power source is secured by rotating the motor at the converted AC voltage.
In contrast, the electric vehicle is an automobile that uses a motor to be driven by a DC power source via an inverter as a power source.
Most of the above-described automobiles are provided with a device for protecting the inverter for driving the motor. Japanese Patent Laying-Open No. 5-15068, for example, discloses an inverter type power source device including a DC power source circuit; an inverter for converting an output from the DC power source circuit into an AC output of a predetermined frequency; an overvoltage detector for detecting an overvoltage state of the DC power source circuit; and a stopper for stopping the operation of the inverter during detection of the overvoltage state of the DC power source circuit.
A switching element constituting the inverter generally includes a semiconductor device such as an IGBT (abbreviating “an insulated gate bipolar transistor”) or a MOSFET (abbreviating “a metal oxide semiconductor field effect transistor”). A dielectric withstand voltage (hereinafter, simply referred to as “a withstand voltage”) of the IGBT or the MOSFET variably depends upon a temperature (such as an ambient temperature or a junction temperature). The withstand voltage of the IGBT or the MOSFET becomes lower as the temperature is lower whereas it becomes higher as the temperature is higher.
However, Japanese Patent Laying-Open No. 5-15068 nowhere particularly discloses fluctuations of the withstand voltage of the inverter element in association with a change in temperature of the inverter element. As a consequence, in the above-described inverter type power source device, it is construed that a level of a DC voltage during detection of the overvoltage state of the DC power source circuit by the overvoltage detector is constant all the time. In a case of such protection of the overvoltage, a voltage in excess of the withstand voltage of the inverter element is applied to the inverter when the inverter is at a low temperature: in contrast, it is construed that the stoppage of the inverter degrades operational efficiency of the inverter even if a voltage sufficiently lower than the withstand voltage of the inverter element is input when the inverter is at a high temperature.